In order to achieve higher peak data rates in future wireless networks, simultaneous transmission and reception of multiple carriers, referred to as carrier aggregation, is considered as a key element. The Third Generation Partnership Project (3GPP) has been standardizing the carrier aggregation for Long Term Evolution (LTE), as described in a paper by G. Yuan, X. Zhang, Wang and Y. Yang, titled “Carrier aggregation for LTE-Advanced mobile communication systems,” published in IEEE Commun. Mag., vol. 48, no. 2, pp. 88-93, February 2010, the disclosure of which is incorporated herein by reference in its entirety. From the perspective of mobile terminals, carrier aggregation poses unprecedented design challenges, especially when multiple, non-contiguous carriers need to be transmitted and received simultaneously.
Regarding the receiver architectures of mobile terminals, a direct conversion receiver is well suited for carrier aggregation. Although direct conversion receiver has gained much popularity recently, as indicated in the book, RF Microelectronics, by B. Razavi, Upper Saddle River, N.J., Prentice-Hall, 1998, the disclosure of which is incorporated herein by reference in its entirety, each carrier inevitably requires an individual receiver, thereby leading to inefficient implementation. On the other hand, a wideband IF double conversion receiver is known, as described in the paper by A. Springer, L. Maurer and R. Weigel, titled “RF system concepts for highly integrated RFICs for W-CDMA mobile radio terminals,” published in IEEE Trans. Microw. Theory and Tech., vol. 50, no. 1, pp. 254-267, January 2002, the disclosure of which is incorporated herein by reference in its entirety. The wideband IF double conversion receiver reuses both the RF mixing stage and IF mixing stage (i.e., local oscillators, or LOs, and mixers), allowing for cost-efficient and power-efficient implementation. Moreover, it retains many of the advantages of the direction conversion receiver, for example, highly-programmable channel selection essential to aggressive carrier aggregation.
The double conversion receiver architecture may be applied to multi-carrier reception. In, e.g., a dual-carrier double conversion receiver, one receiver branch may receive and process a first carrier, while another receiver branch receives and processes a second carrier. By selecting appropriate LO frequencies and mixer parameters, the second receiver branch may share LOs and mixers with the first branch, to conserve hardware.
FIG. 1 depicts a portion of a representative dual-carrier double conversion receiver. In this particular example, the receiver is operative in an LTE downlink employing intra-band, non-contiguous carrier aggregation, although the principles of the present invention are applicable to other receivers and systems. The receiver, in this example, receives a composite, carrier aggregation signal at an antenna, which is low-noise amplified. The receiver includes an RF LO frequency generator and an IF LO frequency generator. An RF down-converter comprises two mixers, driven by two corresponding RF LO frequency signals in Quadrature phase, and low-pass filters. The resulting IF signals are then further mixed with IF LO frequency signals having different phases. The resulting baseband frequency signals are combined in two receiver branches, or signal processing paths. An upper receiver branch receives and process a signal modulated onto a 20 MHz carrier frequency, while a lower receiver branch receives and processes a signal modulated onto a non-contiguous, 5 MHz carrier frequency.
The modulated signals may comply with any communication protocol, such as for example Wideband Code Division Multiple Access (WCDMA). In some embodiments, the communication protocol may comprise Orthogonal Frequency Division Multiplexing (OFDM). As known in the art, OFDM signals are modulated onto a plurality of subcarriers which collectively occupy the carrier bandwidth. As used herein, “carrier bandwidth” refers to the frequency of a carrier onto which one or more signals is modulated. A carrier may comprise a single carrier occupying the specified bandwidth (e.g., a spread-spectrum carrier), or may comprise a plurality of subcarriers which collectively occupy the specified bandwidth.
One drawback to a dual-carrier double conversion receiver such as that depicted in FIG. 1 is the sensitivity to imbalance between the In-phase (I) and Quadrature (Q) components of a received signal (known as IQ imbalance) stemming from the shared, gain- and phase-imbalanced LOs and mixers. IQ imbalance is considered a major impairment source in radio receivers, particularly in OFDM systems.
IQ imbalance arises from imbalanced RF/analog circuitry, which includes LO, mixer, filter, ADC, etc. RF/analog circuitry can be calibrated by controlling some circuit parameters, or the IQ imbalance can be compensated by digital signal processing in the digital baseband. Digital compensation of IQ imbalance has many advantages over analog calibration. For example, in principle, it can work with any radios, since it does not require a dedicated interface between radio circuitry and digital baseband. Major benefits of IQ imbalance compensation are to relax the design requirement on IQ imbalance, and overcome the fundamental limitation of analog components (or, given the same design effort, to improve the receiver performance).
Digital compensation of IQ imbalance is also applicable to double conversion receivers for carrier aggregation, including intra-band, non-contiguous carrier aggregation. One distinction, as compared to conventional IQ imbalance compensation, is the need for joint compensation over multiple receiver branches receiving and processing multiple carriers. This is unsurprising, considering that IQ imbalance causes inter-carrier coupling, such that signals processed in each receiver branch includes its own carrier, as well as the other carriers (i.e., signals processed in other receiver branches).